As far as I can tell, time to read works on all books. I will encounter glitches sometimes when the Kindle sees a chapter break in the middle of a chapter (when a few empty lines occur), or when it does not see any chapters at all.
I don't know yet how a chapter break is designated in a mobi book, or what the conversion process uses to tag a chapter break, but that is probably the source of the few errors that I have encountered.
Someday I will have to look into that, as the accuracy of chapter break locations has become more important with this very nifty feature.